In the manufacture of certain wet-laid paper products, such as facial tissue, bathroom tissue, or paper towels, the web is conventionally subjected to a creping process in order to give it desirable textural characteristics, such as softness and bulk. The creping process involves adhering the web to a rotating creping cylinder, such as the apparatus known as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
The severity of this creping action is dependent upon a number of factors, including the degree of adhesion between the web and the surface of the creping cylinder. Greater adhesion causes increased softness, although generally with some loss of strength. In order to increase adhesion, an adhesive creping aid is used to enhance any naturally occurring adhesion that the web may have due to its water-content, which will vary widely depending on the extent to which the web has been previously dried. Creping aids also should prevent wear of the dryer surface and provide lubrication between the doctor blade and the dryer surfaces and reduce chemical corrosion, as well as controlling the extent of creping.
In order to provide a coating that is durable enough to withstand the mechanical forces under the conditions of heat and/or moisture at the doctor blade, it is desirable to use a resin having some ability to thermoset, i.e., to lose solubility and fusibility by crosslinking. However, the ability to impart wet strength to paper also depends on the thermosetting nature of the resin. In wet-strength grades such as facial tissue, napkin stock, toweling, and the like, it is known that some wet-strength resins can function concurrently as creping aids. However, in non-wet-strength grades such as bathroom tissue, it is desirable to restrict the thermosetting ability of the resin, so as to provide a durable coating on the dryer while limiting the potential development of wet strength in the paper.
An adhesive creping aid, as an aqueous solution or dispersion, is usually sprayed onto the surface of the creping cylinder. Alternatively, it can be added at the wet end of the paper machine, e.g., to the pulp before formation of the sheet, by spraying onto the wet web, etc. Spraying on the dryer surface will minimize the chance that a durable, relatively highly thermosetting resin could transfer to the sheet during creping, to impart possibly undesired wet-strength to a non--wet--strength grade. However, any corrosive ions present in a resin would be concentrated on the dryer surface by this technique. Conversely, adding resin at the wet end of the paper machine would increase the wet-strength development by the more reactive thermosetting resins, but by diluting any corrosive ions in the white water, it would minimize their concentration at the dryer surface.
U.S. Pat. No. 4,501,640 discloses a creping adhesive comprising an aqueous mixture of a polyvinyl alcohol and a water-soluble, thermosetting, cationic polyamide resin that is the reaction product of epichlorohydrin and a polyamide having secondary amine groups derived from a polyalkylene polyamine and a saturated aliphatic dibasic carboxylic acid containing from 3 to 10 carbon atoms. The patent states that the polyvinyl alcohol is necessary to avoid hardening of the resin by diluting it. The resins employed by this patent are thermoserring wet-strength resins, made with relatively high ratios of epichlorohydrin to amine groups in the base polyamide. If they are added at the wet end, they will impart wet strength. If they are sprayed on the dryer surface, they will carry a relatively high content of potentially corrosive chloride ion into the adhesive film.
Canadian Patent 979,579 discloses a creping aid that is the reaction product of epichlorohydrin and a polyaminopolyamide containing secondary amine nitrogen atoms, such as adipic diethylenetriamine polyamide, that has been alkylated with an alkylating agent or with a combination of formaldehyde and formic acid. All of the resins it describes require the additional alkylation process step in their manufacture, and some of its versions have the disadvantage that some residual formic acid or formate ion is carried over to the final resin. Such resins, if sprayed on the dryer surface, would carry residual formate ion as a component of the dryer coating.
U.S. Pat. No. 3,311,594 discloses a wet-strength additive for paper comprising the water-soluble reaction product of epichlorohydrin and a polyamide prepared by reacting a polyamine having at least three amine groups of which at least one is tertiary, with a saturated aliphatic dibasic carboxylic acid containing from 3 to 10 carbon atoms. The reaction product is water soluble but becomes crosslinked and insoluble on drying. The operable polyamines disclosed include methyl bis(3-aminopropylamine), usually referred to as MBAPA. These thermosetting wet-strength resins are made using a ratio of about 0.8 to about 2.0 moles of epichlorohydrin per mole of amine in the polyamide, or preferably about 1.0 to about 1.7 moles of epichlorohydrin per mole of amine. These resins are stabilized for storage by treatment with hydrochloric acid to convert the epoxide functional groups to chlorohydrin groups, which are then dehydrohalogenated to epoxide by an alkali treatment before use. The resins of this patent are thus highly functionalized, effective wet-strength resins, containing substantial amounts of chloride ion under conditions of their intended use. They would impart substantial wet-strength to the sheet if added at the wet end of the paper machine, and could potentially contribute to corrosion problems if applied to a Yankee dryer surface by spraying.
U.S. Pat. No. 3,640,841 discloses the manufacture of paper using as an adhesion aid, a polyaminopolyamide made from an aromatic or aliphatic dicarboxylic acid and a polyalkylene polyamine with two primary amino groups and at least one amino group which is either secondary or tertiary. The resins of this patent are described as having substantially no effect on wet strength of the paper, and as being easily soluble in water. Although their lack of wet strength makes them usable by wet-end addition to grades where wet strength is not wanted, they thus lack the self-insolubilization that is desired for durability of the coating on the Yankee dryer.
It is recognized in the industry that there is a need that is not met by the known compositions for a creping aid that does not contain undesirable residual chemicals and helps to produce paper with the right balance of strength, stretch ability, softness, and absorbency.